1. Field of the Invention
The present invention relates to a liquid delivery system. It applies in particular to the delivery of ultrapure chemicals, especially those intended for the microelectronics industry.
The pressures involved here are relative pressures.
2. Description of the Related Art
The rapid development in the microelectronics industry towards ever greater miniaturization has consequences with regard to the purity of the chemicals used in various phases of the fabrication of integrated circuits. It is now becoming common practice, in the case of chemicals such as hydrogen peroxide, aqueous ammonia and hydrofluoric acid, to specify cation contents of less than 1 ppb (part per billion) and particle contents of less than 500 particles of 0.2 micrometer in size per liter.
These so-called ultrapure liquid chemicals used, for example, in cleaning processes are delivered over and above a certain consumption by centralized delivery systems. These systems comprise the following functions:
withdrawal of the product from a supplier product source, or supply container, to a storage tank, through filtration stages for improving the particulate specifications of the product, possibly with recirculation through the filtration stages in order to improve the particulate specifications of the product while still maintaining the ionic quality; PA1 delivery of the product from the storage tank to a user network via a filtration stage in order to improve the particulate specifications of the product. PA1 a supply container containing a liquid to be delivered, provided with means for maintaining an overhead at an overpressure of less than a first predetermined pressure P1; PA1 an intermediate storage tank provided with means for maintaining an overhead at a predetermined intermediate pressure P2&gt;P1; PA1 means for transferring the liquid from the supply container to the intermediate tank; PA1 at least two delivery containers having a very much smaller volume than that of the intermediate tank, these containers being connected, in parallel, upstream of a liquid outlet in the latter and downstream of a line for delivering the liquid to a user network; and PA1 control means for applying individually to each container either a delivery pressure P3&gt;P2 or a filling pressure P4&lt;P2. PA1 the system comprises three delivery containers connected in parallel; PA1 the transfer means and/or the delivery line are equipped with means for filtering the liquid; PA1 the said maintaining means and the said control means comprise sources of inerting gas, especially nitrogen, these sources being equipped with pressure-regulating means; PA1 the delivery system comprises a line for recycling liquid from the delivery line to the inlet of the storage tank; PA1 the delivery system comprises a line for recycling liquid from the user network to the inlet of the storage tank; PA1 each delivery container consists of a section of vertical pipe closed off at its lower end by a supply and discharge tee and at its upper end by a stopper equipped with an inlet for pressurizing gas; PA1 the pressure P1 is approximately equal to 100 mb and/or the pressure P2 is between approximately 100 and 500 mb and/or the pressure P3 is between approximately 500 mb and 6 bar; and PA1 the volumes of the storage tank and of each delivery container are between 200 l and 5 m.sup.3 and between 1 and 50 l, respectively.
Various means are known for conveying the product from the storage tank. These means use either pumps, or pressure, or vacuum, or else combinations of these means (see, for example, U.S. Pat. Nos. 5,330,072, 5,417,346 and 5,722,447).
These means have certain drawbacks:
Pumped delivery generates particles associated with the pressure variations of the pumps, and the pumps pose reliability problems.
Pressure and vacuum delivery poses reliability problems associated with the incompatibility towards diaphragm valves in a vacuum system, while these diaphragm valves are the only ones compatible with the required purity levels.
Conventional pressure delivery systems use at least two storage tanks of large individual volume, typically corresponding to the daily consumption of the equipment. Typically, the minimum volume of the tanks is 200 l. This requires large cabinet dimensions and the tanks must be able to withstand the delivery pressure, of about 4 bar, or a vacuum. To do this, in the case of corrosive products, the materials used comprise an inner shell made of plastic of the polyethylene (PE), perfluoroalkoxy (PFA) or polyvinylidene fluoride (PVDF) type and an outer reinforcement made of glass fibre or of stainless steel. This tank design can result in ionic contaminations, if the fabrication processes are not perfectly controlled, and safety problems associated with pressurization or with a vacuum in the case of large-volume tanks.